1. Field of the Invention
The present invention is directed to a premixing burner and a method for operating the premixing burner of the invention.
2. Discussion of Background
In view of the extremely low NO.sub.x, CO and UHC emissions specified for the operation of a heat generator, it has become practice to replace diffusion combustion by a premixing distances and, correspondingly, by premixed combustion. A premixing burner of this type is disclosed in EP-0 321 809. The disclosure of this publication involves replacing the conventional premixing distances by a premixing burner which essentially consists, in the flow direction, of at least two hollow partial conical bodies positioned one upon the other, the center lines of these partial bodies extending offset relative to one another. By this means, tangential entry slots are formed along the premixing burner formed in this way and these entry slots are opposite to one another with respect to the flow. There is an airflow through them into the internal space of the premixing burner. The formation of the mixture--from fresh air, possibly enriched by a quantity of recycled exhaust gas, and fuel--for the formation of a combustion airflow takes place in such a way that the premixing burner can have different fuel inlet nozzle arrangements. A first possibility consists in at least one fuel nozzle being provided at the beginning of the premixing burner, i.e. in the region of its smallest cross-section. This fuel nozzle is placed centrally relative to the center lines of the partial bodies extending offset relative to one another. A further fuel inlet nozzle arrangement, which can either be operated individually or is in effective connection with the fuel nozzle previously mentioned, is made available by providing a series of fuel nozzles along the tangential inlet slots at the transition to the internal space. As an example, the injection of a liquid fuel through the centrally placed nozzle takes place in such a way that a conical spray-type fuel column, which does not, however, wet the inner walls of the hollow conical space, forms in the flow direction of the premixing burner. This fuel column is surrounded by the airflow flowing into the internal space--and, if necessary, by a further axially introduced airflow--in such a way that mixture formation takes place within the premixing burner. This mixture is ignited at the outlet from the premixing burner and stabilization of the flame front is induced in the region of this burner mouth by a reverse flow zone which forms there.
If, however, fuels with a high hydrogen content are burned in such a premixing burner, problems occur with the flame stabilization explained above. Due to the higher flame speed of the hydrogen, transition occurs in the burner from premixing operation to diffusion operation. This causes the following problems:
the burner overheats, PA1 the NO.sub.x emissions increase greatly, PA1 pulsations occur in the transition range between PA1 diffusion and premixing operation.